The present invention relates to an image display apparatus that can display color images by using field- or frame-sequential color signals.
FIG. 1 shows a conventional field-sequential image display apparatus, which receives a standard television signal (e.g., of NTSC or PAL system) and converts the standard television signal into field-sequential signals for display. The illustrated image display apparatus comprises a luminance signal input terminal 1, a chrominance signal input terminal 2, low-pass filters 4r, 4g and 4b, storage devices 7r, 7g and 7b, a control signal generator 10, a signal switching device 11 and a display device 12.
A luminance signal representing the brightness, and also defining the resolution, is applied to the input terminal 1. A chrominance signal representing color components is applied to the input terminal 2. Usually, sync signals are added to the luminance signal, and color burst signals are added to the chrominance signal. The luminance signal and the chrominance signal are input to the RGB decoder 3.
The RGB decoder 3 comprises a detector circuit performing a synchronous detection of a chrominance signal using the color burst signal as a reference, and a matrix circuit receiving the outputs of the detector circuit and the luminance signal, and produces three-primary-color signals of red (R), green (G) and blue (B). The R, G and B signals produced from the RGB decoder 3 are band-limited by the low-pass filters 4r, 4g and 4b and are then input to the R, G and B storage devices 7r, 7g and 7b.
The R, G and B storage devices 7r, 7g and 7b store signals of one field (for 1/60 seconds, in the case of the NTSC system). The signals from the storage devices 7r, 7g and 7b are sequentially selected by the signal switching circuit 11, and are then input to the display device 12 for display of color images. The control signal generator 10 is responsive to the sync signals added to the luminance signal, and produces control signals for control over the signal switching circuit 11 and the display device 12.
As disclosed in Japanese Patent Kokoku Publication No. H4-49928, the display device 12 can be configured as illustrated in FIG. 2. The illustrated display device 12 comprises a cathode-ray tube (CRT) for black-and-white display, and polarization filters 53, 55 and 57. The polarization filter 53 has a polarization axis for transmission of red light only, and a polarization axis for transmission of all color components. The polarization filter 55 has a polarization axis for transmission of green light only, and a polarization axis for transmission of all color components. The polarization filter 57 has a polarization axis for transmission of blue light only, and a polarization axis for transmission of all color components. The display device also comprises variable optical retarders (VOR) 54 and 56 in the form of a liquid crystal switch. Each of the variable optical retarders 54 and 56 is selectively turned on or off. When the variable optical retarder (54 or 56) is turned on, the optical retardation of the light transmitting through it is substantially zero. When the variable optical retarder (54 or 56) is turned off, the optical retardation of the light transmitting through it is substantially half the wave length. By selection from the combinations of the operative states of the variable optical retarders 54 and 56, either red, green, blue or white light is output from the selective filter 70, as shown in FIG. 3. That is, when the variable optical retarders 54 and 56 are both on as at (a), only green light is passed or output. When the variable optical retarder is 54 on and the variable optical retarder is off as at (b), only red light is passed or output. When the variable optical retarder is 54 off and the variable optical retarder is on as at (c), only blue light is passed or output. When the variable optical retarders 54 and 56 are both off as at (d), all the color components are passed, so that white light is output.
The variable optical retarders 54 and 56 are respectively controlled by a VOR controllers 59 and 60, such that R, G and B output lights are obtained as shown in FIG. 3. The signals input to the CRT 51 and the control over the polarization filters 53, 55 and 57 are synchronized so as to achieve color image display. A deflection circuit 58 drives the CRT 51. Various control signals from the control signal generator 10 are delivered via a terminal 52 to the respective circuits.
The field-sequential color image display device as described above does not have a shadow mask, and has a high resolution as a small-size color display device.
The storage devices 7r, 7g and 7b store the R, G and B signals, and are used for converting the simultaneous or parallel signals into field-sequential signals. Each of the storage devices 7r, 7g and 7b may comprise, as shown in FIG. 4, an input terminal 41 to which a color signal of R, G or B is input, an A/D converter 42 for converting the input color signal in the form of an analog signal into a digital form, a memory 43 which usually is formed of a semiconductor memory that has a storage capacity of one field of color signals and is in the form of a dual port memory capable of simultaneous input and output, a D/A converter 44 for converting the digital signal into an analog signal and an output terminal 45.
The frequency at which data is written in the memory 43 is different from the frequency at which the data is read from the memory 43. Assume for instance that the display is made using field sequential signals. The field frequency of the NTSC system is 60 Hz. When the R, G and B signals were sequentially selected at 60 Hz, the frequency at which each of the three colors is used for display would be 20 Hz. This lower frequency would cause flicker. To avoid flicker, the R, G and B signals are sequentially selected at 180 Hz, so that each of the R, G and B signals are selected at 60 Hz. The field frequency in this case is 180 Hz. The memory 43 and the D/A converter 44 are accordingly required to operate at a high speed.
FIG. 5A shows the writing in the memory, while FIG. 5B shows the reading and scanning on the CRT 51.
Provided in front of the input 41 to the A/D converter 42 is the low-pass filter 4r, 4g or 4b. This is to band-limit the signals within the Nyquist frequency in order to remove aliasing noises due to sampling.
The above described field-sequential color image display device can be used for a small-size color display device or a projection-type color display device.
However, the color display device described above is inferior to monochromatic display devices, in some respects, in particular, the resolution, the brightness and the power consumption. These inferiority is problematical when, for instance, the display device is used for a monitor (viewfinder) in a video camera.
First, let us consider the brightness of the image. In the above described image color display device, the selective Filter comprising the polarization filters 53, 55 and 57, and the variable optical retarders 54 and 56 is provided in front of the screen of the CRT 51. The output light is inevitably attenuated by these elements. For white light, the light output from the polarization filter 57 is about one tenth of the light emitted from the CRT 51.
Secondly, let us consider the resolution of the image. Since a black-and-white CRT is used, it is theoretically possible to obtain an equal resolution. However, in practice there are limiting factors. First, it is necessary to perform high-rate scanning. As shown in FIG. 5B, the scanning rate is three times that of the scanning speed of the standard television. Scanning at such a high rate is necessary to avoid flicker, as described above. As a result, the frequency of reading from the memory 43 is three times the frequency of writing in the memory 43. There is however a limit in the capacity and the speed of the memory 43. As described above, the input to the memory 43 is band-limited within the Nyquist frequency.
This is explained with reference to FIG. 6A and FIG. 6B. When a signal of a frequency above the Nyquist frequency f.sub.N is applied as shown in FIG. 6A, aliasing noises shown by hatching are created and cannot be removed in the subsequent stages. To prevent the creation of the aliasing noises, the signal needs to be band-limited by the low-pass filter within a frequency one half the sampling frequency. The output of the low-pass filter is as shown in FIG. 6B, and does not have a broad frequency band. On the other hand, when a high-frequency scanning is conducted, the frequency of the signals is raised three times so that the circuit components in the stages after the output of the storage memory, including the drive circuit of the CRT, must have a broad frequency band.
Thirdly, let us consider the power consumption. There are circuit components which are required for color image display, but are not required for the black-and-white image display. That is, the RGB decoder 3, the LPF's 4r, 4g and 4b, and the storage devices 7r, 7g and 7b and the signal switching circuit 11 are required for the color image display, but not for the black-and-white image display. Moreover, the high-speed operation is not required for the black-and-white image display. Thus, the power consumption is larger for the color image display. This is particularly problematical where the color image display device is powered from a battery.
On the other hand, color image display has advantages over the black-and-white image display. For instance, when the color image display device Is used for a viewfinder, it is possible to examine the color fidelity (whether the color of the subject to be imaged is reproduced correctly). Moreover, it is much easier to find or follow a certain subject to be imaged. Furthermore, iris adjustment is easier because the part of the image which is outside the dynamic range loses color.
As has been described with regard to the prior art, because the resolution of the prior art is low, the focus adjustment is difficult. Moreover, because the brightness is low in the prior art, a picture having a high contrast cannot be seen well even if a brightness adjustment is made.